Elastomeric pumps are widely used in healthcare settings to deliver fluids and medication to patients. In some pumps, fluid is stored in a drug reservoir or bladder made of silicon or another rubber polymer. The bladder is attached to a fixed length central support core at positions along the core that are separated by a non-variable distance. When filled, the bladder expands and the increased surface area of the bladder stores energy that exerts pressure on the fluid, driving the fluid out of the bladder. The flow rate of the fluid is often limited by a restricting orifice such as a glass capillary or a section of PVC tubing.
Referring to FIG. 1, a typical flow profile 18 (i.e., flow rate vs. time) for a standard elastomeric pump shows that the flow rate of fluid from the pump is not constant during fluid delivery. Flow begins with a strong initial spike 20 in flow rate, continues with a trough-shaped phase 21 having a lower flow rate, and finishes with a slight second spike 22. While the troughs and peaks of such a profile may be averaged to provide a sufficient flow rate, in some cases, such as for the delivery of toxic medication or when a narrow therapeutic dose is required, the initial spike may result in an overdose or another undesirable situation.
The initial spike 20 is generated by the strong forces exerted on the fluid by the expanded bladder. To mitigate the initial spike in flow rate, a filled pump can be put aside for a waiting period before beginning fluid delivery in order to allow the bladder to lose some of its elasticity, thus reducing the forces exerted on the fluid therein.
The trough-shaped phase 21 results from a combination of two phenomena. As fluid exits the bladder, the bladder contracts and the energy stored in the bladder decreases. Thus, the pressure exerted by the bladder on the fluid decreases, causing the flow rate to drop. At the same time, however, the physical contraction of the bladder results in a thickening of the bladder walls, which causes the bladder to impose more pressure on the fluid. Initially, the first effect is prominent. As the bladder empties, the latter effect becomes progressively more prominent and manifests itself as the second spike 22 at the end of the fluid delivery.
If the thickness of the bladder walls is not uniform, the bladder will expand more rapidly in the thinner regions when receiving fluid, thus further accentuating the thickness variations. This effect causes the expanded bladder to have an asymmetrical shape, which in turn results in an uneven flow rate and variability in flow rate among like pumps. To combat this effect, the bladder is often enclosed in an outer cover that restricts its asymmetrical expansion, such as a rigid cover or a flexible and non-expandable cover. In some pumps, the bladder is formed of a rubber polymer that exerts force on the fluid therein and a silicone lining on the inside of the bladder that prevents the fluid from coming into contact with the rubber polymer.